High power projection oscillograph



Feb. 22, 1938. P.AT. FARNSWORTH ET AL 2,109,289

HIGH POWER PROJECTION OSCILLOGRAPH Filed NOV. 2, 1936 I INVENTORS 5'Pkxlc T T q' 'mmh.$\ SW01- Patented Feb. 22, 1938 UNITED STATES2,199,289 HIGH POWER, PROJECTION osoILLoGRAPH Philo T. Farnsworth, SanFrancisco, and Frank J. Somers, San Jose, Calii'., assignors toFansworth Television Incorporated, San Francisco, Calii'., a corporationof California Application November 2, 1936, Serial No. 108,723

3 Claims.

Our invention relates to projection oscillographs, and'more particularlyto such oscillographs utilizing a luminous screen wherein light isproduced by incandescence rather than by 5 fluorescence.

In cathode ray tubes where the image is produced on a screen by raisingelementary areas thereof to incandescence by ,the impact of a cathoderay beam, the principal difllculties have 10 been first, to obtainsufllcient power to excite the heat screen at low cathode ray gunvoltages, and second, to provide a screen having a sufiiciently low massso that heat losses may be confined almost wholly to radiation.

15 Another difliculty, an outgrowth of the first two above-mentioned, isthat it is desirable to make the picture area of the heat screenrelatively small, preferably of standard motion picture frame size, sothat economical lenses may be used 20 for projection of the image. Whensuch a relatively small screen is used, it is obvious that a very finescanning spot is necessary to obtain high fidelity images, and a finespot is inconsistent with high gun currents.

25 It is therefore the main object of our present invention to provide acathode ray tube of the heat screen type, wherein the gun current can bekept low in. order to obtain a fine scanning spot.

30 Another object of our invention is to provide a high illuminationscreen, even though a small scanning spot is used, and to be able to usea relatively low gun anode voltage.

A further object of our invention is to provid 35 an exceedingly highvoltage on the screen itself, and to generate this voltage close to thescreen and by the use of high frequency oscillators, in order tominimize distortion due to electrostatic stresses.

40 A still further object of our invention is to utilize secondaryemission to increase the number of electrons reaching the heat screen;and again, it is also an object of our invention to feed power to theelectrons bombarding the heat 5 screen after deflection.

Our invention possesses numerous other ob: jects and features ofadvantage, some of which, together with the foregoing, will be set forthin the'following description of specific apparatus 5 embodying andutilizing our novel method. It is therefore to be understood that ourmethod is applicable to other apparatus, and that we do not limitourselves, in any way, to the apparatus of the present application, aswe may adopt various 55 other apparatus embodiments, utilizing themethod, within the scope of the appended claims,

Referring to the drawing:

The figure is diagrammatic and reduced to lowest terms, and comprises asectional view of a preferred tube embodying our invention, togetherwith a sectional view of a preferred oscillator and circuits connectingthe two for operation.

Our novel method of operating a cathode ray tube, together with onepreferred means for practicing the method, may be more fully understoodby direct reierence to the figure.

The cathode ray tube comprises an envelope I provided at one end with awindow 2 and at the other end with a reentrant stem 4. On this reentrantstem we mount a gun heater 5, a gun cathode 6 and a thimble grid 1having a central aperture 8 through which electrons pass to enter thebeam canal 9 of a gun anode l0.

The cathode, grid and anode of the gun are alined to project a beam ofelectrons of low power into the envelope space H and toward aluminescent screen l2 which is preferably formed of refractory materialof low mass, such as, for example, an extremely thin sheet of tantalumor tungsten, or the screen may be knitted from fine refractory wires tohave a mesh smaller than the elementary area it is desired to reproduce,or again, it may comprise parallel rows of refractory Wire spirals; infact, there are many forms in which this screen may be made, the mainfactor being that it shall have low mass so that heat losses may beconfined almost wholly to radiation, and that it shall have anelementary structure such that all desired detail may be reproducedthereon.

A secondary emission screen I4 isprovided between anode 9 and heatscreen l2, preferably closer to the heat screen than to the anode. Theenvelope chamber II between anode 9 and secondary emission screen It ispreferably wallcoated with a silver or nickel film I5, and this wallfilm is connected directly to secondary emission screen M at one end andprovided with an external lead It; at any convenient point. Heat screenI2 is provided with an external lead ll, anode 9 with its lead l9, andthe various gun elements are provided also with their respective leads,as is customary in the art.

For operation, gun heater 5 may be energized from alternating currentmains through transformer 20; gun cathode 6 is connected to ground; gunanode 9 and film l5 are connected together in a combined supply lead 2|;and thimble grid is provided with the usual bias through biasingresistor and battery 22 so that the gun current may be accuratelyregulated.

Scanning oscillators 24 and 25 energize scanning coils 26 and 21 inorder that their fields may move the beam issuing from beam canal 9 intwo directions over secondary emission electrode llfand a projectionlens 29 is provided so that the image formed on heat screen l2 maybeprojected on any convenient viewing surface. In order that thescanning spot or cross section of the scanning beam may be kept as smallas possible, we prefer to surround the beam path with a focusing coil30, energized by focusing battery 3i under the control of resistor 32.

So far we have described all connections to the tube except the voltagesupply to anode l0 and secondary emitter l4, and to the heat screen l2.We prefer to supply voltage to the heat screen from a high frequencyoscillator 35. This oscillator may be any oscillator capable of a highfrequency output, such as three megacycles, for example, although weprefer to utilize a multipactor oscillator for this purpose. Such anoscillator has been described by Philo T. Farnsworth in his applicationfor United States Letters Patent, Serial No. 611042, filed Jan. 27,1936, and comprises an envelope containing a secondary emissive cathode36, a perforated anode'31 and a central ion collector 38.

The multipactor cathode and anode are connected together through a tunedcircuit 39 which forms the primary of a direct-coupled transformer, thesecondary 40 of which is connected to lead l1 and thence to heat screenl2. Multipactor anode voltage is supplied by anode battery 4|, and lead2| is connected to this battery through radio frequency choke 42, thusproviding steady voltage for gun anode ID. The cathode-anode circuit ofthe multipactor contains a blocking condenser 43 to prevent anodevoltage reaching the cathode. Ion collector 38 is connected to groundthrough an ion control source 44 so that ion control electrode 38 isslightly negative to pick up any free ions in the multiplier space.

Such a tube is a self-oscillator at high frequencies, oscillating powerbeing developed by repeated impacts of primary electrons against cathode36, which is preferably treated or otherwise formed in such a mannerthat for each primary impacting it, two to ten secondaries will beemitted. The action of this type of oscillator is well known in the art,due to Farnsworths publications, and no further explanation is deemednecessary.

In operation, we shall describe a particular tube and give the voltageswhich have been found, by experimentation, to be satisfactory in theoperation of that particular tube, and it will be well within theknowledge of those skilled in the art to understand the energization andoperation of other examples.

We prefer to utilize the gun to produce a gun current of about onemilliampere, with the relatively low gun anode voltage of fifteenhundred volts. This means that the beam issuing from beam canal 9 may beof very small diameter and may be kept at a small diameter until itreaches secondary emission electrode M to form an exceeding'ly fine spotthereon. In addition, due to the factthat the gun anode has such a lowvoltage, and because there is a small diameter beam, the scanningsensitivity of the cathode ray beam .to deflection is maintained high.

We then prefer to form the secondary emission screen I in such a mannerthat it will generate copious secondary emission when the electrons inthe beam bombard it, and we have found that we can so treat this screenthat secondaries in the ratio of one to one may be obtained with impactvoltages as low as twenty volts, with a total secondary production foreach primary of as high as ten to one. We form the screen I4 ofrelatively fine wire so that even though the mesh count of the screen I4is greater than the number of elements desired in the image, there isstill plenty of aperture area.

When oscillators 24 and 25 are set in operation, the beam is moved toproduce a picture area on the screen ll. Obviously, some of theprimaries in the beam will pass through the apertures in screen i4without creating secondaries. Other primaries in the beam will, however,impact the wires of screen I4 and generate secondaries. Consequently,there may be available, in the plane of the screen ll, both primariesand secondaries. If, then, oscillator 35 is set into operation so thatthe transformer 39-40 will supply the heat screen l2 with approximatelyfifty to seventy five kilovolts R. M. S. at a frequency of about threemegacycles, both primaries and secondaries will be pulled through screenl4 and tremendously accelerated before they impact heat screen l2. Theradio frequency current applied to the heat screen is, of course,self-rectified within the tube. The frequency of the multiplieroscillator is chosen high enough so that there will be several cycles atleast of the radio frequency per picture element, and there will,therefore, be no matte effect produced in the picture from this source.At the same time, frequencies in the neighborhood of three megacyclesare low enough to allow the use of high Q coils, eflicient oscillatoroperation and eflicient voltage step-up in the transformer 39-40.

Inasmuch as primaries and secondaries are only emitted from screen I atthe point of scansion, it is obvious that only corresponding points onheat screen 12 will be bombarded, and due to the acceleration betweenscreens i4 and I2, as a consequence of the extreme high voltage onscreen i2, the wires of screen I2 at the point of electron impact willbecome incandescent and illuminated in accordance with the modulation ofthe original scanning beam. It is therefore obvious that by supplyingthe majority of power in the tube as an accelerating potential on thefinal screen, the beam itself may be made of sufiiciently low power sothat it may be sensitive to deflection and have an extremely small crosssection, thus eliminating the objections to primary'guns of highpower.

There is, however, another very important feature in the use of a highfrequency alternating potential on screen l2. Screen I2 is adapted to beraised to incandescence by electron impact. It is obvious, therefore,that the mass of the wire from which screen I2 is woven must be small inorder to reduce heat conduction along the wires. and in order that themass may be raised to incandescence without substantial lag. Thematerial 'of the screen may be tungsten or tantalum wire, for example,between .005" and .0005. The type of mesh of the screen may be, andpreferably is, that of a knitted fabric, in order that the individualwires thereof may be longer than the distance between their points ofattachment to the support frame. By utilizing such a knitted screen or asimilar inherently elastic amaaeo mesh, it is possible, as far as thediiferential heat- .ing of the screen is concerned, to maintain thescreen I2. It is obvious that if this high voltage were steady, thatthere would be large electrostatic stresses set up between screen l2 andscreen I4, or between screen l2 and charges on the ad- Jacent tubewalls. Such stresses could only be eliminated by wide spacing betweenadjacent screens, and by the enlargement of the envelope to a pointwhere electrostatic charges on the walls thereof would not be suillcientto place a stress upon screen I2. Such spacings, however, are highlyimpractical, and with spacings demanded by modern design, theelectrostatic stresses set up by a steady potential on screen l2, underthe voltages mentioned, are sufllcient to completely destroy the utilityof screen l2, inasmuch as it is inherently elastic and will distortfreely under stresses much less than those encountered in the examplegiven.

If, however, the screen vI2 is energized by an alternating potential,particularly an alternating potential which is of sufflciently highfrequency so that the screen l2 cannot move under the influence of therapidly changing and reversing electrostatic stresses, it is obviousthat the screen I 2 will stay in its predetermined plane. It is onlynecessary, therefore, to apply an alternating potential having afrequency that is substantially higher than the natural period ofvibration of screen It, and having, in case that the s reen I2 is to beused as an image producer w ere elemental areas are to be formedthereon, the frequency sufiiciently high so that there will be severalt:sycles of the high frequency per picture elemen It is, however, to bedistinctly understood that we also wish to use the method of energizinginherently elastic screens with high frequency potentials in all caseswhere such screens would normally, under the influence of a steadypotential, be subject to electrostatic stresses causing distortion ofthe screen, irrespective of whether or not the screen is to be used as alight source, an accelerating electrode, or in any other fashion withina thermionic tube.

We claim:

1. A cathode ray. tube comprising an envelope containing a source ofelectrons, at collecting electrode normally distortable by theelectrostatic stresses set up when energized to a predetermined steadycollection potential, an oscillator connected to said electrode, andcircuits maintaining the frequency of said oscillator substantiallyhigher than the natural period of said collecting electrode.

2. A cathode ray tube comprising an envelope containing a source ofelectrons, a collecting electrode comprising a fine wire screen normallydistortable by the electrostatic stresses set up when energized to apredetermined steady collection potential, an oscillator connected tosaid electrode, and circuits maintaining the frequency of saidoscillator substantially higher than the natural period of saidcollecting electrode.

3. The method of developing an incandescent image on a heat screen in acathode ray tube which comprises generating a stream of electrons havingan elemental cross section, scanning a picture area with said beam,generating secondary electrons with part of said beam at each elementalarea scanned, combining said secondary electrons with the remainder ofsaid beam, and energizing said screen with an alternating potential at afrequency substantially higher than the number of elements involved.

PHILO T. FARNSWORTH. FRANK J. SOMERS.

